The present invention relates to a vibration prevention support structure used to minimize the vibration of automotive engines, and more particularly to a fluid-damped elastomeric bushing which is effectively applied to, though not limited to, the engine mount of automobiles.
Various types of fluid-damped engine mounts other than a bushing type have been manufactured as a vibration prevention structure for the engine mount. Among them are an engine mount using a partition plate with an orifice and a diaphragm as disclosed in Unexamined Japanese Patent Publication No. 58-170935(1983), another one using a rubber elastic member with one part made thinner and recessed as disclosed in Japanese Patent Publication No. 60-52337(1985) and still another having two compartments divided by a movable wall with an orifice, the movable wall formed, at least partially, by a bellows as disclosed in Japanese Patent Publication No. 58-21131 (1983).
Since these structures have a liquid chamber, the following disadvantages arises: the engine mount is larger than those consisting of ordinary rubber elastic members and fittings; a cover or cap is required for the protection of the diaphragm and the recess; the engine mount support is extended on both sides to avoid interference by the liquid chamber; and a stopper mechanism is required to prevent a large positional deformation of elements in the event that an excess load is applied. All these in turn result in a further increase in the size of the engine mount which occupies a significantly large proportion of the engine room.
In recent years, a front wheel drive is becoming the mainstream of the drive system and the engine mount used for the front wheel drive is mostly of the bushing type having no liquid therein. The engine mount of the bushing type consists basically of an outer sleeve, an inner sleeve and an elastic member. A large displacement applied to the inner sleeve is restrained by the stopper action produced between the inner and outer sleeves, thus ensuring a high level of safety, which is one of the reasons for its use. However, the bushing type engine mount has its disadvantage that the vibration damping characteristic is not as good as that of the above-mentioned large fluid-damped engine mount.
The fluid-damped engine mount other than the bushing type in the vibration prevention support device has a high performance but is suitable only for expensive cars and is large and costly. The bushing type engine mount for the front wheel drive cars is not satisfactory in performance.
If a fluid-damped bushing which is commonly used for vibration damping for vehicle suspension mechanism is used as a vibration damping element in the engine mount, the problem cannot be solved because this will increase the spring constant, deteriorating the comfortable ride.
Any attempt to reduce the rubber hardness or the thickness of the rubber to lower the spring constant will deteriorate the durability, making the anti-vibration structure impractical as the engine mount.
This invention intends to reduce the size of the conventional fluid-damped engine mount, which occupied a considerably large proportion of the engine room, by the use of the fluid-damped bushing which has been difficult to apply to the engine mount structure. For that purpose, the invention reduces the spring constant of the fluid-damped bushing and at the same time gives it a sufficient durability.
An elastic bushing consisting of an elastic member such as rubber interposed between the outer sleeve and the inner sleeve is known. It is also known that to use such an elastic bushing for supporting a heavy object such as an automotive engine, the inner sleeve is put eccentric to the outer sleeve in the direction reverse to load application so that when applied with load the inner sleeve will assume a specified position (almost center position).
Recently, efforts have been made to develop what is generally called a fluid-damped elastomeric bushing of the above eccentric type, in which fluid chambers are formed in the bushing and communicated with each other through an orifice and in which a fluid is sealed in the chambers to effectively damp the vibrations of low frequency. An example is the Unexamined Japanese Patent Publication No. 61-31736(1986).
It has generally been considered that the fluid-damped elastomeric bushing cannot provide a desired damping characteristic when the spring constant is lowered to increase the anti-vibration performance.
Lowering the spring constant requires making thinner the elastic member walls that form the liquid chambers and this, it was believed, in turn would prevent vibrations from producing a change in the volume of the liquid chamber, failing to produce an expected damping effect.
Thus, it was believed that simply making the chamber walls thinner to lower the spring constant is not enough and that as explained in the preceding example a special provision is required to increase the damping performance.
However, the inventors of this invention have found that the fluid displacement is affected not only by the conditions on the compressing side but also by the behavior of the liquid in the chamber being expanded, and also that as long as the volume of the chamber on the expansion side increases during vibration, since an elastic compartment wall is bent toward the interior of the liquid compartment, the compartment is expanded when the bent wall is unbent and a sufficient liquid suction effect is obtained and there is no problem to the liquid transfer but a sufficient transfer of the liquid is accomplished in a smooth manner even when the chamber wall on the compression side is more or less thin. Thus, an expected damping effect is realized.